This invention relates to an optimal acoustic wave propagation orientation on a quartz crystal for surface acoustic wave (SAW) applications including natural single-phase unidirectional transducers.
Surface acoustic wave technology for signal processing has produced a broad range of devices which are used typically in the VHF and UHF range and include filters, resonators, delay lines, convolvers, impedance elements and various other types of devices. Processes for preparing these devices are well known in the art and the device typically involves interdigitated electrodes.
The orientations for surface acoustic wave propagation for any given crystal are completely defined by the Euler angles, lambda, mu and theta. The first two angles lambda and mu, define the crystal cut and the third angle, theta, defines the direction of acoustic wave propagation in that cut. Thus, acoustic wave propagation orientation in the crystal is defined by a unique set of all three Euler angles. See Holstein, H., Classical Mechanics, New York, (1950) (Addison-Wesley).
A theoretical tabulation of surface acoustic wave propagation properties at various orientations for a variety of surface wave materials and particular crystal cuts of interest can be generated and tabulated. A tabulation including velocity, coupling coefficient, electromechanical powerflow, angle curves, and magnitude and phase of the free surface mechanical displacements and electric surface potential can be found in Microwave Acoustic Handbook IA and 2, Slobodnik et al, Air Force Cambridge Research Labs, October 1973 and October 1974. Such data as set forth in these handbooks can be used as a guide to selecting appropriate directions for wave propagation and orientation in the crystal.
The concern in each case in the prior art is to find a crystal orientation which has good coupling between the voltage on the electrodes and the acoustic wave in the crystal itself, low beam steering, and good temperature stability. Applicant has discovered that a quartz crystal having a cut defined by the Euler angles of lambda equal about 45.degree., mu equal about 55.degree. and theta equals plus about 10.degree. to minus about 30.degree., an orientation which has not been heretofor disclosed, provides the needed advantages as set forth above when electrodes are placed on a quartz crystal and in addition, allows the construction of a two electrode per wavelength transducer with unidirectional characteristics. It also allows the construction of a two electrode per wavelength transducer with a desired symmetrical input conductance function and a flat susceptance region. See U.S. Pat. No. 4,599,587, issued July 8, 1986 and entitled Impedance Element for a detailed description of the desired conductance and susceptance.
Thus it is an object of the present invention to provide a quartz crystal having a substantially planar surface defined by the Euler angles lambda equal about 45.degree. and mu equal about 55.degree., and the direction of propagation defined by the angle theta equal plus about 10.degree. to minus about 30.degree..
It is also an object of the present invention to form a surface acoustic wave device comprising two interdigitated electrodes per wavelength on the planar surface of the quartz crystal having a planar surface defined by the Euler angles lambda equal about 45.degree. and mu equal about 55.degree., and the direction of propagation defined by the Euler angle theta equal plus about 10.degree. to minus about 30.degree..
It is still a further object of the present invention to provide the crystal with Euler angles of lambda equal about 45.degree., mu equal about 55.degree. and theta equal plus about 10.degree. to minus about 30.degree. in the form of a wafer for constructing surface acoustic wave devices.